Ellen Baker - Orion Ceramic Studios on wed 21 may 97
This is going to raise a whole lot of hackles with this one!
To avoid S-cracks: (a) Be sure pot bottoms can contract freely during
drying by carefully drawing a taut and level cut-wire between the pot
bottom and the bat or drying board. (b) Turn pots upside down as soon as
they can be safely handled so they may dry as evenly as possible. If you
tend pots carefully through the drying process -- keeping walls and bottoms
"drying together" -- you'll have few (probably NO) cracks of any kind.
All this business about "compressing" (throwing) clay is a bunch of hooey!
Although there may be a little truth to the concept of managing clay
particle alignment/direction, I think it's a shame that so many "experts"
perpetuate the myth that it's possible to significantly "compress" moist
clay between fingers, between fingers and ribs, ribs and bats, whatever --
somehow magically "squeezing water out of the clay". I've often asked,
"So, just how much force does that take?" and, "Where does the water
actually go?" and, "How much water 'disappears'? 1%? 5%?" Not much water
disappears, I can assure you! What a hoax!
There are so many wives tales! Heat rises!!! Clay can be compressed!!!!
Some of pottery's most confident sounding wives' (and/or buck-) tales can
lead folks FAR, FAR AWAY from understanding the real causes and effects of
simple, understandable processes. Some beliefs are so deeply rooted and
steadfastly defended that it can be frightening to challenge them! (I
already sense hackles rising from coast to coast!!!)
There was a good article in "Ceramics Monthly" a few months ago about
wives' tales...it's worth reading, I assure you! Consider re-considering
some of what you've been taught.
/e
Ric Swenson on thu 22 may 97
Teach us what to call it...if not compression what do YOU and those of
great scientific ilk refer to it as? I learn every day, hopefully , and
have been laboring under the impression for a lot of years that there are
several things that will help prevent S cracks. Have I wasted my time with
measures that were just wives' tales. What is the real secret?
Gee, maybe we're CALLING it something different. Maybe compression isn't
the right word... could it be decompression? tension? torsion? fusion?
Fission?? Next you will tell us that "clay doesn't really shrink as it
dries"?.
PUSHING on the bottom of a spinning vessel , as it is "thrown" on the
wheel, DOES remove some of the moisture left there when lubricating the
clay for throwing. (who cares how much moisture it removes? Enough to
affect a change...ok? (Using a rib is just like using the fingers but
you get less wear and tear on the fingers/joints.) The lack of moisture in
the bottom of a cylinder (because it makes drying more even) CAN help
prevent cracks.
Actually, I care little what it is called. PRESSING DOWN (again, who
cares HOW MUCH FORCE it takes? Do you want it to work...or do you want to
measure it? ) on the inside of a vessel helps prevent S Cracks. If you
don't believe that, you obviously have never done much throwing. Or maybe
you throw "upside -down" or use kerosene instead of water to lubricate the
clay. ??
Wives tale or not. COMMON SENSE tells me it works...that and from my own 30
years of DOING it.
Flame that.
Ric
>----------------------------Original message----------------------------
>This is going to raise a whole lot of hackles with this one!
>
>To avoid S-cracks: (a) Be sure pot bottoms can contract freely during
>drying by carefully drawing a taut and level cut-wire between the pot
>bottom and the bat or drying board. (b) Turn pots upside down as soon as
>they can be safely handled so they may dry as evenly as possible. If you
>tend pots carefully through the drying process -- keeping walls and bottoms
>"drying together" -- you'll have few (probably NO) cracks of any kind.
>
>All this business about "compressing" (throwing) clay is a bunch of hooey!
>Although there may be a little truth to the concept of managing clay
>particle alignment/direction, I think it's a shame that so many "experts"
>perpetuate the myth that it's possible to significantly "compress" moist
>clay between fingers, between fingers and ribs, ribs and bats, whatever --
>somehow magically "squeezing water out of the clay". I've often asked,
>"So, just how much force does that take?" and, "Where does the water
>actually go?" and, "How much water 'disappears'? 1%? 5%?" Not much water
>disappears, I can assure you! What a hoax!
>
>There are so many wives tales! Heat rises!!! Clay can be compressed!!!!
> Some of pottery's most confident sounding wives' (and/or buck-) tales can
>lead folks FAR, FAR AWAY from understanding the real causes and effects of
>simple, understandable processes. Some beliefs are so deeply rooted and
>steadfastly defended that it can be frightening to challenge them! (I
>already sense hackles rising from coast to coast!!!)
>
>There was a good article in "Ceramics Monthly" a few months ago about
>wives' tales...it's worth reading, I assure you! Consider re-considering
>some of what you've been taught.
>
>/e
C.T. Wagoner on thu 22 may 97
>To avoid S-cracks: (a) Be sure pot bottoms can contract freely during
>drying by carefully drawing a taut and level cut-wire between the pot
>bottom and the bat or drying board.
I never run a wire under anything. After making 75,000 plus vessels I
rarely have any "S"-cracking.
>All this business about "compressing" (throwing) clay is a bunch of hooey!
We use ribs to compress any wide forms like plates or low bowls. That seems
to be the key to not having the cracking that you speak of. Our clay body
is a high shrinkage fine grained clay. Without the "compressing techniques"
I am convinced we would have a lot of problems. I also try to keep any
excess water out of the bottom from the very start of the process.
cwag@abcs.com
Wagoner Pottery
"Made to be used!"
http://abcs.com/cwag/
Jack Troy on thu 22 may 97
If, according to Ellen Baker, it's a myth that compressing the clay causes
s-cracks to go away, but the cracks really DO go away after the clay is
compressed, well, viva la myth!
Jack Troy
Karl David Knudson on thu 22 may 97
On Wed, 21 May 1997, Ellen Baker - Orion Ceramic Studios wrote:
> ----------------------------Original message----------------------------
> This is going to raise a whole lot of hackles with this one!
>
> There are so many wives tales! Heat rises!!! Clay can be compressed!!!!
Heat doesn't rise? You're not one of the architects that decided that
putting electric coils in the ceiling of a single story building was an
efficient way to heat a house, are you? Regardless of the fact that the
sheet of aluminum in the floor supposedly "draws" heat down. The simple
fact that my head would be a balmy 85 degrees while my feet were turning
blue from the frost on the bathroom floor proved to me that this was a
dumb idea. Not to mention the $100 per month electric bill when the
outside temp was 40, and the thermostat was set to its lowest setting. I
had always thought that this "brilliant idea" was a plot by the local
electric companies in the 60's to ensure that the eugene/springfield area
consumed gigawatts of power. This is true, there are MANY houses/duplexes
in our area that were built this way. I've experienced two of them!
Sorry, I'm rambling, this touched a nerve. If you want to determine what
heat really does here's a simple test. Light a match and hold your finger
under it (to see if heat sinks), then next to it on each of the four sides
(to test if it migrates to the north, S, E, W), lastly place your finger
above the match (to see if it rises). The position where you get burned
is where the heat goes.
As for ceiling heat, it is only slightly more intelligent than the two
story out-house.
Karl in eugene, who heats with the DeLongi oil heaters now. $35/ month
and the house is warm.
Vince Pitelka on thu 22 may 97
>This is going to raise a whole lot of hackles with this one!
Ellen -
It is evident that "raising hackles" was your primary goal in this post.
You could not possibly be serious about your comments. If you do not
believe that heat rises, then you probably have trouble with gravity as
well. And in ten years of teaching, when students do not compress the
bottoms of their pots adequately, they get S-cracks. When they do compress
adequately, no S-cracks.
It is pretty simple. When widening the bottom of a pot, clay is usually
pulled away from that which remains as the bottom thickness, and no
compression occurs. When raising the walls, the clay is compressed from
both sides. Obviously if nothing is done about this inequity, the clay in
the bottom shrinks more than the walls, and VOILA, S-cracks. I said it was
simple.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Phone - home 615/597-5376, work 615/597-6801
Appalachian Center for Crafts
1560 Craft Center Drive, Smithville TN 37166
Jonathan Kaplan on fri 23 may 97
>----------------------------Original message----------------------------
>This is going to raise a whole lot of hackles with this one!
>
>To avoid S-cracks:
>All this business about "compressing"
>
>There are so many wives tales!
> Consider re-considering
>some of what you've been taught.
>
etc. etc
I'd like to take issue with Ellen Baker's assertions in the above post,
edited, for space as well as misinformation. Raising hackles is one thing,
providing misinformation on this list is another.
Caveat: while there are ways we have learned that work for us, these are
indeed only ways that work for each of us. And if it breaks the rule or
provides results for you, hey, I'm all for it. Present your information as
information that may work for you at a particular time. Present it is dogma
or the rule and you are in pretty deep. This is not intended as a personal
attack or as a flame, only as part of my endeavor to promote a conscious
atempt to keep the misinformation off this list. Misinformation serves as
a higly distructive and counter prodcutive effort. While we all learn from
our mistakes and those of others, lets put a damper (no pun intended) on
information that becomes confusing, wrong, and of littel or no purpose.
Having made pots for well into thirty years by most of the available
methods, I'll assert that
one of the most crutial elements that contribute to the success of work
made by any forming method is particle alignment. How the particles are
stacked, compressed, or other wise, will dramatically have a cause, affect
and effect relationship to the successful production of your wares. Its
that simple.
Jigger your ware with the pug oriented incorrectly, the resuts are often s
cracks. Press your pots with a pug charged in the die incorrectly, and
you'll get huge s cracks. Throw the pot, slab the pot, extrude the pot, all
will react, dry and fire as a direct result of the forming technique used
with particular attention the how the particles are stacked or aligned.
Once again, read Lawrence's "Ceramic Science for the Potter" for a very
well presented and
understandable explanation of particle alignment vis a vis each
manufacturing method.
Please make a conscious attempt to present information correctly.
Jonathan
Jonathan Kaplan jonathan@csn.net
Ceramic Design Group Ltd./Production Services
PO Box 775112
Steamboat Springs CO 80477
(970) 879-9139*voice and fax
http://www.craftweb.com/org/jkaplan/cdg.shtml
http://digitalfire.com/education/articles/kaplan1.htm
Gavin Stairs on fri 23 may 97
I too am somewhat puzzled by this compression business. But lets see what
can be:
S-cracks are tension cracks. That means something is pulling the clay
apart where the crack appears. There are two things I can think of that
would do this.
The first is that the clay where the s-crack appears is wetter than the
surrounding clay. This could well happen in the bottom of a pot if it is
not well sponged out. But it would not distinguish a hump thrown from a
lump thrown pot. It might be worse on cut off pots versus bat thrown and
uncut pots, but that is not usually done, except on plaster bats.
The second thing is the stress of opening the clay. If you do this by
pushing sideways, you are pulling the clay away from the center, and the
bottom of the pot will be in tension. In a pot thrown right on the wheel
head, the head will tend to resist this. However, on the hump, the clay
beneath the pot won't resist the pull, and the bottom of the pot will have
a remnant tension when it is cut off. During drying, this might result in
s-cracking.
However, if you open by pressing down into the clay, thus forcing the clay
to extrude sideways, then the remnant stress in the clay at the bottom of
the pot will be comressive, and the pot will be much more resistant to
s-cracking.
If you open by pushing in and then pulling sideways, as is almost
universally true, then it is probably wise to go back to the center and
push down, so as to convert the remnant tension into compression. Thus,
compression is probably more than a wives tale. But note that it is the
last thing you do to the clay that counts, not anything before. You can
even compress after the pot is cut off, or leather hard, for instance when
you are burnishing, turning or trimming. In the latter cases, you will
have to push harder to achieve the proper result.
The compression is not to dewater, or make the clay more dense. It is to
alter the residual stress state of the clay. To reprogram its memory, so
to speak.
Some people have commented on the weak state of clay that has been
overstressed. This is a state similar to wedged clay. If a pot goes all
wet and flaccid, stop whatever you are doing and let it rest for a while.
Then take it up again, adjust the wetness, and try again. You will have to
accomodate for the fact that the whole clay body has now become stiff, but
it will be even all over. This will permit you to continue throwing, or
forming by other means.
Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
Bob Hanlin on sat 24 may 97
At 12:12 PM 5/21/97 EDT, you wrote:
>All this business about "compressing" (throwing) clay is a bunch of hooey!
Well, I really don't know about the "clay particle" hoey, but I do know
that since I've started compressing the bottoms of my pots I've had almost
no "s" cracks (large bowls with small thick bottoms are an exception) and I
handle them just as you prescribe. Some of 'em crack anyway. But the
other stuff almost never cracks (can't remember my last "s" crack in a vase
or mug,etc.) and I let it dry off masonite bats. Now I guess that you'll
tell me that the bat substitutes for your method. I would suggest that
you've got a bit to learn. But maybe you know it all...
Paul Monaghan on sat 24 may 97
Karl David Knudson wrote:
>
> ----------------------------Original message----------------------------
> On Wed, 21 May 1997, Ellen Baker - Orion Ceramic Studios wrote:
> > ----------------------------Original message----------------------------
> > This is going to raise a whole lot of hackles with this one!
> >
> > There are so many wives tales! Heat rises!!! Clay can be compressed!!!!
>
> Heat doesn't rise? You're not one of the architects that decided that
> putting electric coils in the ceiling of a single story building was an
> efficient way to heat a house, are you? Regardless of the fact that the
> sheet of aluminum in the floor supposedly "draws" heat down. The simple
> fact that my head would be a balmy 85 degrees while my feet were turning
> blue from the frost on the bathroom floor proved to me that this was a
> dumb idea. Not to mention the $100 per month electric bill when the
> outside temp was 40, and the thermostat was set to its lowest setting. I
> had always thought that this "brilliant idea" was a plot by the local
> electric companies in the 60's to ensure that the eugene/springfield area
> consumed gigawatts of power. This is true, there are MANY houses/duplexes
> in our area that were built this way. I've experienced two of them!
>
> Sorry, I'm rambling, this touched a nerve. If you want to determine what
> heat really does here's a simple test. Light a match and hold your finger
> under it (to see if heat sinks), then next to it on each of the four sides
> (to test if it migrates to the north, S, E, W), lastly place your finger
> above the match (to see if it rises). The position where you get burned
> is where the heat goes.
>
> As for ceiling heat, it is only slightly more intelligent than the two
> story out-house.
>
> Karl in eugene, who heats with the DeLongi oil heaters now. $35/ month
> and the house is warm.
Friends,
Lots of misunderstanding in this one. Heat doesn't rise or sink it
mearly flows from regions of higher temperature to regions of lower
temperature. What rises/sinks etc is the medium which has been heated.
As for electric coils in the ceiling - they are heating objects in the
room via infrared energy. It does work if you have absorbing, darker
colored materials, furnishings, etc in the room.
Enough Thermodynamics unless someone is dying for more.
Paul :-))
--
Paul J. Monaghan email: paul@web2u.com
WEB2U Productions --- http://www.web2u.com
The "COOLEST" Site on the WEB
"The Computer Secrets are hidden at http://www.web2u.com/secret"
"HOMO CLAYARTUS" is alive at http://www.web2u.com/clayart
Karl David Knudson on sun 25 may 97
On Sat, 24 May 1997, Paul Monaghan wrote:
> ----------------------------Original message----------------------------
> As for electric coils in the ceiling - they are heating objects in the
> room via infrared energy. It does work if you have absorbing, darker
> colored materials, furnishings, etc in the room.
How many people would paint their rooms a dark color and put dark
furnishings in them. Mankind moved out of caves thousands of years
ago...
Sure it "works" but how efficiently? I COULD heat my house by
building giant furnaces in my yard and heating the surrounding air. A
balmy 72 year round. Of course I would consume hundreds of millions of
dollars of energy each year, but who cares, I'm warm right? SUB and EWEB
would love me.
It would probably be cheaper to spray everything with Nils's Heat Utility
Coating. Cheaper still to have electric space heaters on the floor.
cheaper still to have built the heating colis in the floor to begin with.
We are all laughing about this right?
Karl in eugene
David Hendley on mon 26 may 97
Karl in eugene:
I've been laughing all week. Its better than crying about the time
I've spent
reading about hair-spliting sematics.
David Hendley
Maydelle, Texas
"Change is inevitable, except from vending machines"
Karl David Knudson on tue 27 may 97
On Sun, 25 May 1997, Paul Monaghan wrote:
Our proir ramblings deleted.
> Infrared ceiling heat AKA
> Ceilheat works, works well, and is very efficient.
I'm curious about your definition of efficient. Based upon my own
observations Ceilheat was anything but. I'm definging efficiency in
terms of comfort per $$$. Ceilheat does work. It melted everything
meltable in our cupbords (mostly chocolate :( ), the ceiling area was hot,
the floor was cold, I slept at night with a sheet, a wool blanket, a
foam blanket, a down comforter, a blend comforter and an afgan, and my
feet were still very cold. To get up in the morning was well frigid to
say the least, yet at the same time when I stood up my head was warm.
Our local winter temp. is about 20-40 F. so we're really not even
that cold. My monthly electric bill jumped $85 when we turned the ceil
heat on. (I rent half of a duplex, my neighbors went up almost $100.)
When I turned off the breaker to the ceiling heat and plugged in 2 1500
watt DeLongi electric oil heaters and a 1000 watt space heater instead my
monthly bill dropped $35-60 per month. Surely space heaters are not more
efficient than built in heating right? The house was also comfortably
warm, something it wasn't with the Ceilheat.
> The IR heats your body, clothing and objects in the room but not the air
> directly. Therefore, you can feel warm and comfortable even though you
> are maintaining a lower air temperature.
Isn't there also a property of radiation that addresses radiation's
weakening over distance? Isn't it something like the fraction of the
square of the distance... I don't remember. It's why that infrared bulb
feels warm when you put your hand next to it as opposed to 10 feet
away.
> So go ahead and laugh but
> trust me when I tell you that forced hot air is the most inefficient,
> dirtiest, least uniform form of building heating.
Do you have ceilheat Paul?
Hoping that everyone's not taking this too seriously, the only thing
really worth crying about was my old electric bill.
Karl in Eugene.
Richard Burkett on wed 28 may 97
Wow! I go away for a few days and amazing things get posted!
Following some basically good advice Ellen Baker says:
>All this business about "compressing" (throwing) clay is a bunch of hooey!
> [ranting against "experts" and more deleted]
>"So, just how much force does that take?" and, "Where does the water
>actually go?" and, "How much water 'disappears'? 1%? 5%?" Not much
water
>disappears, I can assure you! What a hoax!
Well Ellen, I hope you're kidding. I'm not sure where you've gotten your
information on this, but the responses that I've seen so far on ClayArt
seem to be running pretty much against your statement. It's hardly a hoax.
Particle sizes, spacing and alignment are important. What happens when
clay is deflocculated? Where does all that water come from to make clay
liquid slip when you have added the same amount of water that would be in
an normal plastic clay? I'd like to offer a few more comments.
First of all, if you don't think compression has an effect on clay
shrinkage I offer the following experiment to all of you. It's easy to do,
and although it uses slabs of clay, the forces on the clay (compressing,
stretching) are similar to those used in throwing the bottom of a pot.
You'll also know the drying and firing shrinkage of your clay when you're
done.
My results on porcelain (a worst case scenario as clays go) surprised me.
Porcelain shrinkage (total shrinkage from wet clay)
Normal clay handling
drying bisque cone 10
5.5% 5.5% 12.0%
compressed
2.0% 2.0% 11.0%
stretched
7.5% 7.5% 15.0%
As you can see the results were dramatic - enough to cause substantial
cracking or warping.
Shrinkage tests
1. Roll out a small slab of clay of normal wetness on a cloth or canvas,
being careful to roll it with a rolling pin a all directions
("compressing" the clay equally). Carefully cut at least three 2x6 inch
(5x15 cm) rectangles for test tiles, but don't lift them off the canvas
yet. Remove all the extra clay around them and flip the clay and cloth
over (clay side down) onto a flat board or bat. Flip them back over onto
another board without bending them (use a board/clay/board sandwich) if
you want to use the smooth rolled side. Carefully peel the cloth off the
backs of these test tiles without warping or stretching the tiles (this is
important!).
Work quickly so the tiles don't dry substantially during this whole
process.
2. With one tile, carefully grasp just the very ends and gently try to
stretch it just a bit longer (the 6 inch direction) without seriously
distorting or tearing the tile.
3. With a second tile, press on the short ends and try to make it just a
little bit shorter than 6 inches. Use a couple of pieces of wood to push
if it's easier.
4. Leave the third tile undisturbed.
5. Mark ALL tiles with both what you did to them in steps 2-4 (compressed,
stretched, normal), AND shrinkage marks. Mark them with the type of clay
used, too, for furture reference if you like. For the shrinkage marks,
make a ruler and draw a thin straight line down the length of the tile and
place two thin perpendicular marks EXACTLY 10 cm apart. Using 10
centimeters (100 milimeters) makes it easy to read the results as a
percentage. DON'T MOVE THE TILES.
6. Let the tiles dry normally. When they are dry, take your metric ruler
and carefully measure how much less than 10cm (100mm) it is between the
shrinkage marks. This is the drying shrinkage. For example if the marks
are now 94mm apart, they are 6mm shorter than 100mm (100 - 94 = 6) and the
drying shrinkage is 6%. Record this or scratch it carefully into the tile
if you like.
7. Fire the tiles normally, recording the resulting shrinkage after each
firing.
NOTE: for greatest accuracy of results do several identical tests for each
(normal, compressed, stretched) and average the results.
Richard Burkett - School of Art, SDSU, San Diego, CA 92182-4805
E-mail: richard.burkett@sdsu.edu <-> Voice mail: (619) 594-6201
Home Page: http://rohan.sdsu.edu/dept/rburkett/www/burkett.html
CeramicsWeb: http://apple.sdsu.edu/ceramicsweb/index.html
HyperGlaze@aol.com & http://members.aol.com/hyperglaze/
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